Conventionally, a wire saw is known as means for cutting a workpiece such as a semiconductor ingot into slices. The wire saw comprises a plurality of guide rollers, a motor that rotationally drives the guide rollers, and the like, wherein a cutting wire is wound between the guide rollers to form a wire group in which a large number of the cutting wires are arranged. With this wire saw, a workpiece is cutting-fed in a direction perpendicular to the wire axial direction with respect to the wire group formed by the wire which is run at high speed in an axial direction, thereby being cut into a large number of wafers at the same time.
For example, the wire saw described in Japanese Patent Application Laid-open No. 2000-61805 is known as a wire saw of this type. The wire saw described in Japanese Patent Application Laid-open No. 2000-61805 comprises a first workpiece cutting unit and a second workpiece cutting unit (main roll units) each having a plurality of guide rollers, and a pair of wire feeder/winders, wherein a single cutting wire fed from one of the wire feeder/winders is passed around and stretched between guide rollers of each workpiece cutting unit and wound by the other wire feeder/winder. A wire group for cutting a workpiece is thus formed in each of the workpiece cutting units.
Generally, as to a wire saw, it is important to maintain tension in a cutting wire at an appropriate level. However, with a wire saw including two workpiece cutting units such as that described in Japanese Patent Application Laid-open No. 2000-61805, an effect of a cutting operation by a workpiece cutting unit on an upstream side of a wire running direction of the wire saw causes a fluctuation in tension in a cutting wire introduced into a workpiece cutting unit on a downstream side of the wire running direction.
As means for suppressing the fluctuation in tension, Japanese Patent Application Laid-open No. 2000-61805 discloses a tension detector that detects tension in a cutting wire between both workpiece cutting units and a tension adjustment mechanism that controls the tension based on a value detected by the tension detector. The tension adjustment mechanism includes a pair of dancer rollers across which the cutting wire is passed around and stretched and an arm which is pressed against the cutting wire between the dancer rollers to thereby vary the tension in the cutting wire, being operated so as to maintain tension in a wire introduced into a downstream-side workpiece cutting unit at an appropriate level.
However, in this wire saw, the tension adjustment mechanism is activated only upon fluctuation in the cutting wire tension detected by the tension detector fluctuates, thus hardly obtaining high responsiveness.